1. Field of the Invention
This invention relates to a newspaper for offset printing, specifically a newspaper for offset printing that offers high surface strength, with improvements in the piling of paper powder on the blanket and the problem of Neppari (over-adhesion), the latter of which is caused by the adherence of paper to the blanket, at the time of offset printing.
2. Description of the Related Art
Recently newspaper printing has experienced a rapid conversion from the conventional relief printing system to the offset printing system, and it is said that more than 80% of today's newspaper production employs offset printing. In the offset printing system a plate, usually referred to as a PS (pre-sensitized) plate, is made, whereupon a dampening solution and ink are supplied to the plate for printing. The plate is a flat plane on which the portion to be printed is processed to serve as a lipophilic surface and the portion not to be printed is processed to serve as a hydrophilic surface. When a dampening solution and ink are supplied to the plate, the ink adheres to the portion to be printed and the dampening solution to the portion not to be printed. The ink is then transferred from the plate to the paper via the blanket.
In offset printing where ink with a relatively high degree of tack is used, printing paper having high surface strength is needed. Also, since a dampening solution is added to the paper surface at the time of printing, if paper having low surface strength or low water resistance is used, paper powder will accumulate on the blanket or become mixed in with the ink, thereby causing the so-called “blur” problem on the print surface. Furthermore, with the weight reduction of the newspaper the demand for improved printing opacity is increasing, and as a means of improving paper opacity white carbon, titanium oxide, talc and other inorganic pigments are increasingly being used as fillers in papermaking. However, these inorganic pigments easily ooze out of the paper layers along with the dampening solution, being one of the paper-powder elements that may pile up on the blanket. Moreover, the increased content of ink-free pulp leads to an increase in ash content in the paper, which may actually worsen the blanket-piling problem.
To address these problems during offset printing, the surface of the paper sheet used for newspaper printing has conventionally been coated with surface-treatment agents containing water-soluble, high-molecular materials such as starch, polyvinyl alcohol or polyacrylamide. These surface-treatment agents increase the strength of the paper surface and help the micro-fiber and fillers on the paper surface to adhere firmly to the sheet of paper. On the other hand, if the application amount increases, the adherence of the paper surface also increases when moistened, causing the so-called “Neppari problem”, which is the adherence of the paper to the blanket and may result in the tearing of the paper during printing. Moreover, as a result of using the surface-treatment agents in large amounts, the infiltration of ink into the paper surface is suppressed, which can easily lead to uneven saturation of the ink during color printing (i.e. unevenness in color intensity) due to uneven ink application.
Conventionally, various methods have been proposed to increase the surface strength of paper while reducing its adherence. For example, Japanese Patent Application Laid-open No. 8-13384 discloses a method to reinforce the paper surface strength by coating the surface with a specific polyacrylamide compound and simultaneously increase the water-resistance of the paper surface using polyvalent aldehyde. In Japanese Patent Application Laid-open No. 5-59689, a method to improve the sizing properties, strength and adherence of the paper surface by coating the surface with water composite in which polyvinyl alcohol is mixed with block copolymer of ethylene oxide and propylene oxide has been proposed. In either case, however, neither a reduction in paper-surface adherence nor an increase in surface strength has been sufficiently accomplished, and uneven saturation of the ink arising during color printing has gone unsolved.
Furthermore, the generation of paper powder has conventionally been suppressed through the use of a rosin-emulsion sizing agent and other internal sizing agents to increase the sizing degree of the paper surface and thereby prevent a dampening solution from infiltrating to the inner layers of the paper during offset printing. However, the aforementioned internal sizing agents easily lead to the problem of extensive foaming due to white water in high-speed paper machines such as those used to make newspaper. Moreover, mechanical pulp, in which the sizing effect is difficult to ascertain, is often used as a raw material for newspaper. Therefore, if a retention-aiding agent is used together with the internal sizing agents, the pitch within the papermaking system will be taken into the paper, diminishing the whiteness of the newspaper.